1. Field of the Invention
The present invention relates to a device for adjusting the amount of exposure for use in copying machines, the original reading device of facsimile systems, etc.
2. Description of the Prior Art
In copying machines and the like, an illumination system as shown in FIGS. 1 or 2 is used for illuminating the surface of an original in the form of a slit so as not to produce a shadow on the object (original) when the object is a book or the like having an indentation.
The illumination system comprises a tubular light source lamp a, a main reflecting mirror b and a subreflecting mirror c. The system has a first illumination nation optical path p for reflecting the light from the light source lamp a at the main reflecting mirror b and concentrating the light on the surface of the original d, and a second illumination optical path q for concentrating the light from the lamp a on the original surface d upon reflection at the mirror b and the subreflecting mirror c.
The machines of a particular type have a specified ratio between the amount of light projected upward obliquely rightward in the drawing onto the original through the first path p and the amount of light projected upward obliquely leftward on the original through the second path q.
On the other hand, the tubular light source lamp a used is a halogen lamp or the like which varies longitudinally thereof in the amount of light emitted, while the amount of light reflected at the main reflecting mirror b or the subreflecting mirror c also varies longitudinally thereof. It is therefore difficult to give the slit-like illumination area on the original surface d a uniform illuminance distribution longitudinally of the area.
It is further known to provide a movable light blocking member A in the first optical path between the reflecting mirror b and the original surface d for adjusting the amount of light through the path P longitudinally of the slit-like illumination area as seen in FIG. 1. Similarly, a known movable light blocking member B is disposed in the second optical path q between the main reflecting mirror b and the subreflecting mirror c for adjusting the amount of light through the path q longitudinally of the slit-like illumination area as shown in FIG. 2.
However, either of these conventional arrangements is adapted to adjust the amount of light through only one of the first optical path p and the second optical path q, resulting in a variation in the light amount ratio. Thus, even if the overall amount of exposure is controllable to a predetermined value, the arrangement locally upsets the balance between the amounts of right and left portions of the light reaching the original surface d. In the case of copying machines, for example, this gives rise to the problem of producing a shadow on the copy locally.